The present invention relates to a new and improved method of, and apparatus for, manufacturing a foundry mold, especially for compacting foundry molding material.
In its more particular aspects, the present invention specifically relates to a new and improved method of, and apparatus for, manufacturing a foundry mold from a foundry molding material which is poured or pneumatically infed into mold frame means containing mold pattern means including one or more mold patterns, a molding frame and a filling frame. Subsequently, the foundry molding material poured or pneumatically infed into the mold frame means is compacted by compacting means, for example, by any of compressed-air surge compacting means, combustion-force surge compacting means, pressure compacting means, vibrational compacting means or combined pressure-and-vibrational compacting means. During such compacting operation the entire or at least a predominant portion of the foundry molding material present in the filling frame is displaced into the molding frame.
The strength or stability of foundry molding materials, particularly of bentonite-bonded sand molds is achieved by compacting the foundry molding material which has been loosely poured or pneumatically infed into the mold frame means. As the most important compacting operations there are considered the operations of jarring, jolting or vibrating; jarring, jolting or vibrating in combination with squeezing or pressing, suction pressing, blast pressing, high-pressure pressing and, for a number of years to an increasing degree, surge compacting using compressed air or the combustion gas obtained by ignition of combustible gas mixtures.
Increasing demands on the technical quality of castings or cast workpieces result in increasing requirements concerning the technical quality of the mold containing the foundry molding material. As an example of such castings or cast workpieces which progressively become more complicated, there are mentioned herein cast elements or members used in the automotive industry. In particular there are desired high dimensional precision and fine surface finish which can only be ensured by mold frame means containing uniformly compacted foundry molding material having a uniform density distribution and a uniform strength or stability distribution throughout the mold packing. However, all known compacting methods encounter fundamental difficulties which are caused by the poor pourability of clay-bonded foundry molding materials and by the partially very significant differences in the height and volume of the mold patterns which are placed in the mold frame means. Furthermore, the pattern plates tend to be more and more densely packed with mold patterns for economical reasons and conjointly therewith the distance or spacing between the individual mold patterns as well as the spacing between such mold patterns and the wall of the mold frame means is reduced to an ever increasing extent. As a result, the foundry molding material encounters progressively increasing difficulties for thoroughly shaping or molding the foundry molding material disposed in the intervening spaces in the presence of a sufficient compacting pressure and so as to assume or develop sufficient strength or stability at such locations or critical regions.
Presently used mold manufacturing means are capable of producing very high compacting pressures, however, do not achieve conjointly therewith uniform compaction throughout the foundry molding material. Frequently, and quite to the contrary, the non-uniformity of the packing density of the foundry molding material and its particles or grains and thus also the non-uniformity of the dimensional stability within one and the same mold, in fact, tend to increase with increasing compacting pressure. These effects are caused by laterally directed forces which increasingly occur at high compacting pressures in the foundry molding material above large-area mold patterns which cause strong dam-up or blockage effects. Such lateral forces result in plug formation, that is in the formation of highly compacted bridges of the foundry molding material which extend between the intermediate mold regions and render difficult the access of the foundry molding material and the compacting pressure into the cleft-shaped or slot-shaped regions therebelow. Due to such lateral forces, there is also particularly impaired the edge or peripheral compaction between the mold patterns and the wall of the mold because in the related circumferentially extending zone of the sand mold containing the foundry molding material the effective compacting pressure head is diminished in any case due to the friction existing between the foundry molding material and the wall of the mold frame means. As a consequence, the packing of the foundry molding material is not sufficiently supported in the mold frame means and this results in the extremely feared offset or shift phenomena. Deep-level packing portions frequently are poorly shaped or molded to such an extent that such deep-level packing portions may already rupture during the mold pattern withdrawal or may yield during the subsequent casting operation, thus producing dimensional imprecisions in the casting or cast workpiece. Further faults like edge wear, erosion and penetration are also the consequence.